I wonder if there is anyone in this group who has experience o
the single-crystal and its phasing control as fitted to RXs prior
to 1950?

AIUI, the phasing control is adjusted so that the frequency that would give
the audio image is phased out by being in the notch.

If that is true, how is it then possible to adjust the BFO to a
pleasant-for-you tone,
because such adjustment will alter the CIO frequency?

Surely the BFO has to be set in advance to be halfway between the serial
resonant
frequency and the notchable parallel resonant frequency?

This is a query stimulated by my current project which is to build a
boatanchor style
RX, with a flywheel-loaded slide rule dial like that of the Eddystone EA12